Sensor element and gas sensor with enhanced waterproofing

A sensor element includes an element body having a measurement-object gas flow section formed therein, and a porous protective layer arranged to cover first to fifth surfaces of the element body. When an external wall that is the thinnest of parts of an external wall which constitute the element body and extend from the measurement-object gas flow section to the first to fifth surfaces is defined as a thinnest external wall and a surface corresponding to the thinnest external wall is defined as a closest surface, a part of the protective layer which covers the closest surface overlaps the entirety of the thinnest external wall when viewed in a direction perpendicular to the closest surface, and has one or more internal spaces formed therein which overlaps 80% or more of the thinnest external wall when viewed in the direction perpendicular to the closest surface.